This invention relates to a silver halide photographic material containing at least one silver halide emulsion that has enhanced light absorption. The invention is directed in particular to a color photographic material with high sensitivity, reduced granularity and radiation sensitivity.
Increasing the sensitivity of silver halide based imaging composites to reflected scene information offers the potential for improved photographic performance. Examples of how increased response to light within the visible spectrum can provide system specific benefits include: (i) shorter exposure times which in turn enables xe2x80x9cstop-actionxe2x80x9d image capture; (ii) increased depth of field enabled by the use of a smaller effective aperture; (iii) or improved penetration of artificial illuminants (so-called xe2x80x9cflash distancexe2x80x9d).
However, as executed to date, examples of silver halide based image capture media which display increased sensitivity have suffered from at least two major problems. The first relates to increased noise (or xe2x80x98graininessxe2x80x99) associated with the conventional tactic of increasing detector (emulsion crystal) size to elevate sensitivity. The second area of dissatisfaction related to existing highly light sensitive photographic materials is the rapid and significant reduction in signal-to-noise {S/N} response as these composites are exposed to normal background radiation outside the visible spectrum. The combination of these two facets of existing high speed emulsions can yield inferior results under normal usage conditions, severely compromising the utility described above.
In general, useful photographic sensitivity is correlated with the size of the most light sensitive silver halide detector (emulsion) employed. For tabular emulsions, the operative size variable is surface area per crystal, which in turn is a direct function of the equivalent circular diameter (ECD). Unfortunately, noise, as measured by micro-scale density variation or granularity, is also directly correlated with ECD. Equally distressing is the observation that sensitivity to radiation (outside the visible spectrum) is also a direct function of ECD, with larger ECD having greater sensitivity to and increased damage from radiation. Typical damage imparted from radiation exposure includes: (i) reduced discrimination due to increased minimum density (D-min); (ii) reduction in intended sensitivity to visible light and (iii) increased granularity.
Conceptually, one potential solution which simultaneously addresses both concerns associated with high speed photography would be to provide equivalent sensitivity (or speed) with silver halide crystals of smaller ECD. Central to this goal involves enhancing the amount of spectrally specific light absorbed by the crystal.
J-aggregating cyanine dyes are used in many photographic systems. It is believed that these dyes adsorb to a silver halide emulsion and pack together on their xe2x80x9cedgexe2x80x9d which allows the maximum number of dye molecules to be placed on the surface. However, a monolayer of dye, even one with as high an extinction coefficient as a J-aggregated cyanine dye, absorbs only a small fraction of the light impinging on it per unit area. The advent of tabular emulsions allowed more dye to be put on the grains due to the increased surface area per mole of silver. However, in most photographic systems, it is still the case that not all of the available light is being collected.
The need is especially great in the blue spectral region where a combination of low source intensity and relatively low dye extinction results in a deficient photo response. The need for increased light absorption is also great in the green sensitization of the magenta record of multilayer color film photographic elements. The eye is most sensitive to the magenta image dye and this layer has the largest impact on color reproduction. Higher speed in this layer can be used to obtain improved color and image quality characteristics. The cyan layer could also benefit from increased red-light absorption that could allow the use of smaller emulsions with less radiation sensitivity and improved color and image quality characteristics. For certain applications, it may be useful to enhance infrared light absorption in infrared sensitized photographic elements to achieve greater sensitivity and image quality characteristics.
One way to achieve greater light absorption is to increase the amount of spectral sensitizing dye associated with the individual grains beyond monolayer coverage of dye (some proposed approaches are described in the literature, G. R. Bird, Photogr. Sci. Eng., 18, 562 (1974)). One method is to synthesize molecules in which two dye chromophores are covalently connected by a linking group (see U.S. Pat. No. 2,518,731, U.S. Pat. No. 3,976,493, U.S. Pat. No. 3,976,640, U.S. Pat. No. 3,622,316, Kokai Sho 64(1989)91134, and EP 565,074). This approach suffers from the fact that when the two dyes are connected they can interfere with each other""s performance, e.g., not aggregating on or adsorbing to the silver halide grain properly.
In a similar approach, several dye polymers were synthesized in which cyanine dyes were tethered to poly-L-lysine (U.S. Pat. No. 4,950,587). These polymers could be combined with a silver halide emulsion, however, they tended to sensitize poorly and dye stain (an unwanted increase in D-min due to retained sensitizing dye after processing) was severe in this system and unacceptable.
A different strategy involves the use of two dyes that are not covalently linked to one another. In this approach the dyes can be added sequentially and are less likely to interfere with each other. Miyasaka et al. in EP 270 079 and EP 270 082 describe silver halide photographic material having an emulsion spectrally sensitized with an adsorbable sensitizing dye used in combination with a non-adsorbable luminescent dye that is located in the gelatin phase of the element. Steiger et al. in U.S. Pat. No. 4,040,825 and U.S. Pat. No. 4,138,551 describe a silver halide photographic material having an emulsion spectrally sensitized with an adsorbable sensitizing dye used in combination with a second dye that is bonded to gelatin. The problem with these approaches is that unless the dye that is not adsorbed to the grain is in close proximity to the dye adsorbed on the grain (less than 50 angstroms separation) efficient energy transfer will not occur (see T. Fxc3x6rster, Disc. Faraday Soc., 27, 7 (1959)). Most dye off-the-grain in these systems will not be close enough to the silver halide grain for energy transfer, but will instead absorb light and act as a filter dye leading to a speed loss. A good analysis of the problem with this approach is given by Steiger et al. (Photogr. Sci. Eng., 27, 59 (1983)).
A more useful method is to have two or more dyes form layers on the silver halide grain. Penner and Gilman described the occurrence of greater than monolayer levels of cyanine dye on emulsion grains, Photogr. Sci. Eng., 20, 97 (1976); see also Penner, Photogr. Sci. Eng., 21, 32 (1977). In these cases, the outer dye layer absorbed light at a longer wavelength than the inner dye layer (the layer adsorbed to the silver halide grain). Bird et al. in U.S. Pat. No. 3,622,316 describe a similar system. A requirement was that the outer dye layer absorb light at a shorter wavelength than the inner layer. A problem with previous dye layering approaches was that the dye layers described produced a very broad sensitization envelope. This may be desirable for some black and white photographic applications, but in a multilayer color film element this would lead to poor color reproduction since, for example, the silver halide grains in the same color record would be sensitive to both green and red light.
Yamashita et al. (EP 838 719 A2, U.S. Pat. No. 6,117,629) describes the use of two or more cyanine dyes to form more than one dye layer on silver halide emulsions. The dyes are required to have at least one aromatic or heteroaromatic substituent attached to the chromophore via the nitrogen atoms of the dye. Yamashita et al. teaches that dye layering will not occur if this requirement is not met. This is undesirable because such substitutents can lead to large amounts of retained dye after processing (dye stain) that affords increased D-min. Similar results are described in U.S. Pat. No. 6,048,681 and EP 1,061,431A1. EP 1,061,411A1 describes forming dye layers by using dyes with additional polycyclic rings. The dyes have at least one heterocyclic ring that has two or more additional rings attached to it. This may promote dye-dye interactions by increasing van der Waals forces, however, adding hydrophobic, aromatic rings to the dye molecules is undesirable in that the dyes are more likely to be retained after processing and give higher dye stain.
Yamashita and Kobayashi (JP 10/171058) describe silver halide photographic emulsions that contain an anionic dye and a cationic dye, where the charge of either the anionic dye or the cationic dye is 2 or greater. Tadashi and Takashi describe (JP2001013614 A) combinations of cyanine dyes wherein the logP for the dye combination is in a certain preferred range.
Further improvements in dye layering have been described in U.S Pat. No. 6,143,486, U.S. Pat. No. 6,165,703, U.S. Pat. No. 6,329,133, U.S. Pat. No. 6,331,385, and U.S. Pat. No. 6,361,932. Useful antenna dyes (dyes in the the outer layer of the multilayer) for dye layering that have less dye stain after processing were described in U.S. Pat. No. 6,312,883.
Photographic origination materials for general use require sufficient latitude of exposure to record scenes of widely varying luminance; especially for use in simple cameras with fixed exposure used under a wide variety of lighting conditions. It is well known in the art to provide an element with two, three, four or more layers in each color record containing silver halide emulsions of successively different sensitivity in order to provide the desired exposure latitude. At the same time, the emulsion characteristics must be carefully chosen such that the overall characteristic curve relating density to exposure of the material, which is the combined response of the individual layers, is linear in order to reproduce the original scene faithfully.
At the same time high sensitivity and fine image structure are demanded in the marketplace. In addition, the fine image structure must be maintained during the life of the product prior to photographic processing despite the impact of high energy background radiation.
The emulsions sensitized by the dye-layering technique can be designed to provide higher sensitivity to the fastest layer or they may be used to obtain high speed with a decreased grain size and granularity improvement when employed in the most sensitive layer. However, the use of dye-layered emulsions in the most sensitive element can have significant, detrimental consequences. In particular, it is to be anticipated that enhanced absorption of light by the dye-layering technique relative to an emulsion of similar speed not featuring the dye-layering technique will optically retard participation of underlying layers of similar spectral sensitivity. None of the existing prior art provides insight as to how to compensate for this aspect of dye-layering emulsions.
It is well known to provide the fastest layer with less than a stoichiometric amount of coupler to reduce the unwanted, continued contribution of the largest grains to granularity in high exposures. This results, however, in a concomitant reduction of the latitude of the fastest layer. A choice of a larger grain size emulsion for the layer of second highest sensitivity would result in increased granularity.
Allway et al. U.S. Pat. No. 6,319,660 disclose a speed-enhancing compound (development promoting agent or DPA) with a minimum of three heteroatoms and a Clog P sufficient to increase the photographic speed of the photographic material compared to the same material without the compound. Unknown in the art, however, is how or if this technology has any beneficial utility in unison with dye-layered emulsions.
The problem remains to provide a silver halide photographic element having a combination of fastest and intermediate layers with a linear characteristic curve, high speed, and low granularity.
In one embodiment this invention provides a silver halide photographic element comprising a support bearing a cyan dye image-forming unit comprised of at least one red-sensitive silver halide emulsion layer having associated therewith at least one cyan dye-forming coupler, a magenta dye image-forming unit comprising at least one green-sensitive silver halide emulsion layer having associated therewith at least one magenta dye-forming coupler, and a yellow dye image-forming unit comprising at least one blue-sensitive silver halide emulsion layer having associated therewith at least one yellow dye-forming coupler, wherein at least one of the dye image forming units contains layers of differing sensitivities, and the layer of highest sensitivity contains a development promoting agent and a silver halide emulsion comprising tabular silver halide grains having associated therewith at least two dye layers comprising (a) an inner dye layer adjacent to the silver halide grain and comprising at least one dye, Dye 1, that is capable of spectrally sensitizing silver halide and (b) an outer dye layer adjacent to the inner dye layer and comprising at least one dye, Dye 2, wherein the dye layers are held together by more than one non-covalent force; the outer dye layer absorbs light at equal or higher energy than the inner dye layer; and the energy emission wavelength of the outer dye layer overlaps with the energy absorption wavelength of the inner dye layer.
In another embodiment this invention provides a silver halide photographic element comprising a support bearing a cyan dye image-forming unit comprised of at least one red-sensitive silver halide emulsion layer having associated therewith at least one cyan dye-forming coupler, a magenta dye image-forming unit comprising at least one green-sensitive silver halide emulsion layer having associated therewith at least one magenta dye-forming coupler, and a yellow dye image-forming unit comprising at least one blue-sensitive silver halide emulsion layer having associated therewith at least one yellow dye-forming coupler, wherein at least one of the dye image forming units contains layers of differing sensitivities, and the layer of highest sensitivity contains a coupler represented by Formula IIIa 
where R7 is an alkyl, aryl, alkyloxy or aryloxy group, R8 is a coupling group, and R9 is an alkyl or aryl group containing at least 8 carbon atoms; and a silver halide emulsion comprising tabular silver halide grains having associated therewith at least two dye layers comprising (a) an inner dye layer adjacent to the silver halide grain and comprising at least one dye, Dye 1, that is capable of spectrally sensitizing silver halide and (b) an outer dye layer adjacent to the inner dye layer and comprising at least one dye, Dye 2, wherein the dye layers are held together by more than one non-covalent force; the outer dye layer absorbs light at equal or higher energy than the inner dye layer; and the energy emission wavelength of the outer dye layer overlaps with the energy absorption wavelength of the inner dye layer.
Surprisingly, it has been found that the elements of the invention have high sensitivity, a linear characteristic curve, and low granularity.